Soybean productivity is below average regional productivity due to poor implements of improved technologies, poor extension contact, input scarcity and untimely input supply; marketing also constrained by seasonality, low access to market information and price fluctuation. The specific objective includes mapping of soybean value chain, analyzing marketing margins and analyzing determinants of soybean market supply in the study area. Multistage sampling procedure was used to select representative households. Both qualitative and quantitative data types were used. Data were collected from both primary and secondary sources. Primary data were collected from 136 farmers, 38 soybean traders, and 30 consumers. Descriptive statistics and econometrics models (OLS model) were used to analyze the collected data. Major VC actor includes union, office of agriculture and natural resource, Assosa agricultural research center, smallholder farmers, brokers, local collector, retailer, primary cooperative, wholesaler and consumers. Eight market channels were identified for soybean value chain. Total gross marketing margin (TGMM) was highest in channel VIII (Producer–primary cooperative–union– wholesaler1–wholesaler2) which was 30.3% and lowest in channel III (Producer–local collector–retailer–consumer) which was 12%. Producer’s share (GMMp) was highest in channel III which account 88 % because of the involvement of few intermediaries and they sell at farm gate to reduce the transportation cost and lowest in channel VIII which is 69.6%. Union obtained the highest profit share in channel III which is 24.1% because union mostly purchase soybean produced for seed purpose. From traders wholesaller1 grasp the highest share of marketing margin in channel VI which is 18.6%. Primary Cooperatives obtained smallest profit in channels VII which is 3.3%. The OLS model result indicated that Quantity of soybean market supplied significantly influenced by five variables; these are family size, land holding, credit access, farming experience and cooperative membership. Therefore, policy aiming at strengthening the linkage among soybean value chain actors and value addition, increase connection with research institutes and other supportive institutions, increasing soybean production, expanding accessibility of market infrastructure and encouraging awareness creation through extension service is recommended. Key words: Soybean, Value chain analysis, Bambasi, Ordinary list square, Marketing margin.